Based on our results, we propose a scenario
which is sketched in Figure REF .
In this scenario, central galaxies in halos of \(\log (M_{\rm h}/\, h^{-1}{\rm M_\odot })<12\) 
are mostly star-forming galaxies [1]},
and thus contain large amounts of cold gas.
Since the number of satellites per halo
in these halos is small, most of the galaxies
exhibit no significant AGN activity.
When halos grow to about \(10^{12}\, h^{-1}{\rm M_\odot }\) , the number of
satellites reaches the order of unity, and close interaction
happens within a time scale \(t_{\rm it}\sim 3\)  Gyrs. Interaction makes the
cold gas in the disk flow into the galaxy center,
triggering AGN activities and strong star
formation (or even a star burst).
The AGN associated with the star burst usually
has a high accretion rate and luminosity [2]}, [3]},
and thus can launch strong winds [4]}
that can quench the star formation in galaxy center and shut off
the fuel supply to the AGN. Because of this,
the stellar population formed in the star burst
evolves from \(D_n4000=1\)  to 1.5 within about
1 Gyr. During this period, secular non-axisymmetric
structures produced by the interaction
may continue to send cold gas from the disk,
albeit at a reduced rate, to feed the SMBH,
producing a low-level AGN. In this case, low
level AGNs may be triggered multiple times
by the secular evolution, so that the total
duty cycle time is much longer than that of
a single cycle. Since the total amount of cold
gas is already reduced by the star burst,
the AGNs triggered by the secular evolution
are expected to be weak, and no significant
enhancement of star formation is expected
from the process. As the process proceeds,
the host galaxy will become poorer in cold gas,
eventually becoming quenched in star formation,
and the AGN triggered by the secular structure
will become too weak to detect. During this
long time scale, halos can still grow
significantly while galaxy mass grows little,
which may explain why quiescent galaxies
have more massive halos than star-forming galaxies
of the same stellar mass.
